1. Field of the Invention
This invention relates to a prosthetic intervertebral disc nucleus. More particularly it relates to an artificial disc nucleus made of a hydrogel material that exhibits diminished lateral bulging at certain sections of the nucleus and/or diminished narrowing under high compressive load.
2. Description of the Prior Art
The intervertebral disc is a complex joint anatomically and functionally. It is composed of three structures: the nucleus pulposus (hereinafter referred to as the "natural nucleus"), the annulus fibrosus and the vertebral end-plates.
Degeneration of an intervertebral disc, often due to aging, is believed to be a common cause of back pain. An additional cause of back pain is displacement of an intervertebral disc. In either of these cases, the nucleus pulposus may herniate and/or protrude into the vertebral canal or intervertebral foramen. Most herniations and protrusions occur on the posterior or posterior/lateral side of the disc where the disc annulus is generally thinner and weaker than on the anterior side. The disc may in turn press upon the spinal nerve that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution.
Degenerated or displaced discs may be treated by replacing the entire natural disc with an artificial disc. Various artificial discs are disclosed in the following U.S. Pat. Nos.: 3,867,728 to Stubstad et al.; 3,875,595 to Froning; 4,309,777 to Patil; 4,714,469 to Kenna; 4,349,921 to Kuntz; 4,772,287 to Ray et al.; 4,904,260 to Kuntz; 4,911,718 to Lee et al; and 4,932,969 to Frey et al. (All of the documents cited herein, including the foregoing, are hereby incorporated in this application in their entireties.)
Alternatively, degenerated or displaced discs may be treated by replacing only the natural nucleus rather than the entire disc. U.S. Pat. No. 5,047,055 to Bao et al. relates to a prosthetic nucleus for a vertebral disc. The prosthetic nucleus is made of a hydrogel material which, when dehydrated, has a dimension of less than half of that of the hydrated hydrogel nucleus. It may be implanted in the dehydrated state, and when fully hydrated it has a size and shape generally conforming to a natural nucleus. The shaped hydrogel has a much higher structural integrity than the natural nucleus because it has shape memory due to cross-linking or strong hydrogen bonding in the polymer matrix. However, it still exhibits extensive lateral bulging under high compressive load if there is no boundary to constrain the deformation. As used in this application, lateral bulging means bulging that occurs along the side perimeter of the nucleus.
Normally, insertion of a shaped hydrogel prosthetic nucleus does not require removal of the disc annulus or end-plates so lateral bulging of the hydrogel nucleus is restricted by the restoring force of the stretched annulus fibers and the external force of the end plates. Also, due to its structural integrity, the hydrogel nucleus will generally not herniate or bulge through previously herniated areas or through incisions which were made to remove the degenerated nucleus. However, in some cases incisions or previously herniated areas will be particularly large, or the restoring force of annulus fibers will be diminished due to fiber damage. In such cases it is important that lateral bulging of a shaped hydrogel be diminished in order to prevent further herniation or fiber damage.